Hello. I'd like someone experienced to suggest me a good second distortion pedal. The one I have is Boss MT-2. It's versatile, when I combine it with overdrive on my Valvestate amp I can get a really heavy tone. However, I can't get those "dirty", really high-gain tones out of it (think early Slipknot). If anyone could suggest me a pedal that could give such a tone, combined with amp overdrive or not, I'd be grateful.
I'm not certain if I'm thinking the same as you but if I'm not mistaken, a lot of the sound you're going for comes from running the amp itself hard. It's very difficult to get a pedal to sound like a raging amp all on it's own. Based on my limited understanding of gear, distortion pedals tend to be hard clipping whereas overdrives are soft clipping. Many distortions have a very saturated gain structure. I think you may be looking for the high gain, just not the saturation when you say "dirty". If that's the case, look into some overdrive pedals. Lots of good ones out there. The Tubescreamer types are very popular and Ibanez and Maxon both make them. From my experience I would avoid the strikingly popular Zakk Wylde Overdrive by MXR. I did buy one and found it to be noisy and worked more like a distortion pedal (hard clipping) so it didn't work well leading into a gain channel on my tube amp. The OD I currently have that I could not function without is the MI Audio Tube Zone. It's not cheap but it does everything I've wanted in an OD and more so it's incredibly tweakable and can really match the amps voicing so it doesn't seem to color the amp's tone.

So yeah, try looking into some overdrives since they can also boost a gain channel on your amp. In my experience soft clipping stuff will give you more of what you're looking for. Also, try EQing differently, maybe even look into a pedal based EQ. The MXR 10 Band is pretty killer and really helps carve your sound to your liking.